Neutron spectrometry can involve the generation of a polychromatic beam of neutrons which are characterized by different velocities and hence energies, these velocities and energies being associated with respective frequencies.
Generally a monoenergetic beam of neutrons is desired for use, for instance, in the investigation of solid matter by neutron scattering or for activation of a substance or for investigation of the dynamic behavior of atoms.
It is thus a common practice to subject the polychromatic beams to a form of filtration or monochromatization, selecting the neutrons of a single energy velocity or frequency from the polychromatic beam. Such monochromatization or energy-separation of the polychromatic beam of neutrons is generally carried out by drum separators and disk choppers, as described in commonly owned U.S. Pat. No. 4,099,102 issued July 4, 1978.
In time-of-flight neutron spectrometers it is important to control the rotation angle of the rotor of a synchronous motor relative to the input frequency of the frequency generator energizing the motor (i.e. the degree to which the rotor may be lagging the position determined by the applied frequency) or to control relative angular positions of the rotors of several synchronous motors.
In a known system for controlling the angular position of a rotor, a pulse train is fed to a cascade of dividers, the effective length of the chain being temporarily modified for implementing an incremental change in the rotor's position. The disadvantage of that system is that a pulse of immoderately high frequency must be generated. If a lower frequency is used, the minimal incremental change in rotor angle correspondingly increases.
In another known system, the original pulse train is converted by an integrator into a sawtooth signal fed to a threshold detector whose output signal is a rectangular wave shifted in phase with respect to the original pulse train, the magnitude of the shift depending on a preset triggering level. A major disadvantage of this method is the limitation of phase shifts to 20.degree. in the control of the rotor position of three-phase six-pole synchronous machines.
Reference may also be had to German Patent Document (open application--Offenlegungsschrift) 27 55 796 and 15 37 160 and the documents or references of record in U.S. Pat. No. 4,099,103.